Application of radon-222 to investigate groundwater discharge into small shallow lakes Natasha T. Dimova a,b,⇑ , William C. Burnett a , Jeffrey P. Chanton a , Jane Elizabeth Corbett a a Department of Earth, Ocean and Atmospheric Sciences, Florida State University, Tallahassee, United States b Department of Geological Sciences, University of Alabama, Tuscaloosa, United States article info Article history: Received 11 January 2011 Received in revised form 11 January 2013 Accepted 22 January 2013 Available online 18 February 2013 This manuscript was handled by Laurent Charlet, Editor-in-Chief, with the assistance of Christophe Tournassat, Associate Editor Keywords: Radon-222 Groundwater discharge TMDLs (Total Maximum Daily Loads) Small shallow lakes Seasonal groundwater discharge study Florida abstract The objective of this work was to assess the groundwater discharge component of the hydrological bud- gets of several small lakes in north and central Florida using 222 Rn (radon, t 1/2 = 3.8 days) as a quantitative groundwater tracer. Methane gas and conductivity were also used as secondary natural tracers to help locate active sites of seepage. We present results based on a steady-state radon mass balance model developed to assess groundwater discharge into shallow unstratified lakes. Model assumptions include a constant 222 Rn input flux over relatively short (days–weeks) periods and a well-mixed water column. Detailed surveys in several Florida lakes using continuous 222 Rn measurements supported these assump- tions. In five of the seven studied lakes a high to moderate groundwater inflow was detected using this technique, while in two of them the discharge was very low to not detectable. Based on these results and the size of the lakes the calculated water-renewal times based on groundwater inflows were in a range between 3 and 40 months. Close 2-year examination of the groundwater dynamics at two of the lakes that showed substantial discharge, did not show great seasonal groundwater discharge variations. These radon-derived groundwater seepage fluxes agreed well with seepage meters and water budget calcula- tions performed independently for some of the lakes. The approach proved to be very efficient, relatively inexpensive, and should be able to be applied as a routine procedure for estimating groundwater dis- charge for similar lakes elsewhere. The ultimate purpose of our efforts is to use this method in obtaining important information for the hydrological budget of these lakes that will allow evaluating nutrient load- ings and TMDLs (Total Maximum Daily Loads) calculations. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Surface–groundwater exchange may play a major role in the ecological status of some lakes. Groundwater inputs, for example, often determine the trophic status of lakes (Hayashi and Rosenber- ry, 2002). Although the volume of a groundwater contribution may be relatively small compared to surface sources, the concentrations of the solutes derived via groundwater could be orders of magni- tude higher than surface inputs and thus could have disproportion- ally greater effects (Johannes, 1980; Moore, 2010; Null et al., 2012). In some cases, the so-called ‘‘seepage lakes’’ for example, ground- water seepage is the only significant source of external nutrient loading besides direct atmospheric deposition. According to a re- port prepared by the U.S. Geological Survey, about 70% of Florida’s lakes are seepage lakes with no natural stream inflow or outflow (USGS, 2002). The drainage basin of a seepage lake commonly is re- ferred to as a closed basin because of the lack of surface water out- flow from the basin; however, there is outflow from the basin through groundwater seepage. Therefore, hydrological and nutri- ent budgets of most Florida lakes should take into account inputs and loadings from diffusive sources such as groundwater. In spite of its potential importance, groundwater sources have typically not been estimated adequately. The main reason is that groundwater flow into lakes is often patchy and diffusive and is thus difficult to evaluate using traditional methods. Modeling ef- forts based on water budgets often suffer from a lack of detailed and relevant hydrologic data. The research presented here was conducted to provide information to assist the Florida Department of Environmental Protection’s (FDEP) efforts to establish appropri- ate nutrient loading values due to groundwater discharge under the TMDL (Total Maximum Daily Load) program for several lakes in north and central Florida. Our approach uses radon ( 222 Rn) as a geochemical groundwater tracer. Radon is a good groundwater tracer because it is very con- centrated in subterranean waters compared to surface waters. In addition, new technology has allowed automated and continuous measurements of radon-in-water (Burnett et al., 2001; Dulaiova et al., 2005). The lakes selected for this study are situated in north 0022-1694/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2013.01.043 ⇑ Corresponding author. Current address: Department of Geological Sciences, University of Alabama, 2030 Bevill Bldg., Tuscaloosa, AL 35487, United States. E-mail address: ntdimova@as.ua.edu (N.T. Dimova). Journal of Hydrology 486 (2013) 112–122 Contents lists available at SciVerse ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol